Unsafe attributes

Summary

• The following attributes must now be marked as unsafe:
  • export_name
  • link_section
  • no_mangle

Details

Rust 1.82 added the ability in all editions to mark certain attributes as unsafe to indicate that they have soundness requirements that must be upheld.¹ The syntax for an unsafe attribute looks like this:

#![allow(unused)]
fn main() {
// SAFETY: there is no other global function of this name
#[unsafe(no_mangle)]
pub fn example() {}
}

Marking the attribute with unsafe highlights that there are safety requirements that must be upheld that the compiler cannot verify on its own.

Starting with the 2024 Edition, it is now required to mark these attributes as unsafe. The following section describes the safety requirements for these attributes.

Safety requirements

The no_mangle, export_name, and link_section attributes influence the symbol names and linking behavior of items. Care must be taken to ensure that these attributes are used correctly.

Because the set of symbols across all linked libraries is a global namespace, there can be issues if there is a symbol name collision between libraries. Typically this isn't an issue for normally defined functions because symbol mangling helps ensure that the symbol name is unique. However, attributes like export_name can upset that assumption of uniqueness.

For example, in previous editions the following crashes on most Unix-like platforms despite containing only safe code:

fn main() {
    println!("Hello, world!");
}

#[export_name = "malloc"]
fn foo() -> usize { 1 }

In the 2024 Edition, it is now required to mark these attributes as unsafe to emphasize that it is required to ensure that the symbol is defined correctly:

#![allow(unused)]
fn main() {
// SAFETY: There should only be a single definition of the loop symbol.
#[unsafe(export_name="loop")]
fn arduino_loop() {
    // ...
}
}

Migration

The unsafe_attr_outside_unsafe lint can update these attributes to use the unsafe(...) format. The lint is part of the rust-2024-compatibility lint group which is included in the automatic edition migration. In order to migrate your code to be Rust 2024 Edition compatible, run:

cargo fix --edition

Just beware that this automatic migration will not be able to verify that these attributes are being used correctly. It is still your responsibility to manually review their usage.

Alternatively, you can manually enable the lint to find places where these attributes need to be updated.

#![allow(unused)]
fn main() {
// Add this to the root of your crate to do a manual migration.
#![warn(unsafe_attr_outside_unsafe)]
}